In a wireless communication system, there is a possibility of occurrence of a frequency offset due to Doppler effect while the user terminal (mobile station) is traveling at a high speed wirelessly communicates with a wireless base station (hereinafter simply called “base station”), for example, a user in SHINKANSEN uses a cell phone. Such a frequency offset may largely affects a quality of receiving signals at a user terminal.
For example, in communication between a user terminal serving as a receiver and a base station serving as a transmitter that are in line of site (LOS) through the use of a direct wave, a Doppler frequency appears in the form of a frequency offset of a received signal, and the offset is observed to be an amount of phase rotation on the baseband signal.
Such a frequency offset can be eliminated by an AFC (Automatic Frequency Control) circuit included in the receiver. An AFC circuit estimates a frequency offset (an amount of phase rotation) with reference to a received signal, and provides an amount of phase rotation in the reverse direction of the estimated amount of phase rotation so that the frequency offset can be eliminated.
One of the known methods of estimating a frequency offset uses a signal, called a pilot signal and a reference signal, commonly known to both receiver and transmitter. For example, as illustrated in FIG. 15, there is a known method in which an amount of phase rotation is estimated on the basis of correlation value of reference signals received at different timings.
The range of an estimatable frequency in this method of estimating depends on the cycle (intervals) of transmitting the reference signal. For example, estimating a large frequency offset caused from a Doppler frequency during high-speed traveling requires the reference signal to be transmitted at a shorter cycle.    [Non-Patent Literature 1]p. Moose, “A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction”, IEEE Trans. Commun., vol. 42, no. 10, October 1994    [Non-Patent Literature 2]M. Morelli and U. Mengali, “An Improved Frequency Offset Estimator for OFDM Applications”, IEEE Commun. Lett., vol. 3, no. 3, March 1999    [Non-Patent Literature 3]H. Cheon, “Frequency Offset Estimation for High Speed Users in E-UTRA UPLINK”, Proc. PIMRC 2007
In the base station, a downlink (DL) carrier frequency fc obtained by correction in the AFC circuit of a user terminal, serving as the receiver, is followed by an uplink (UL) carrier frequency fc+fu. The term “fu” represents an amount of an offset between the UL and the DL carrier frequencies.
For the above, on an UL signal received by the base station, twice 2df the one-way Doppler shift df for the DL and the UL is superimposed (see FIG. 14). Accordingly, the base station receives a signal containing a doubled frequency offset 2fd corresponding to the traveling speed of the user terminal (receiver).
This requires the base station to have a capability of estimating a frequency offset in a wide range and removing the estimated offset. However, in conventional techniques, the range of an estimatable (compensatable) frequency offset (phase rotation) depends on the cycle (intervals) of transmitting a known signal, such as the reference signal, and is therefore limited.